Acute shortage of potable water in some areas of the world is a problem for which sea water is seen as an answer. Conversion of it to fresh water by various desalination processes, however, is plagued by a technological problem resulting from the nature of the inorganic salts contained in the sea water. They have low aqueous solubility and their precipitates can hinder the mechanism or manner of the particular desalination process used, examples of such effects being the blockage of semipermeable membranes of an osmotic process and the reduction of heat transfer of a distillation process. This will lower the efficiency of the desalination and ultimately cause such deterioration of performance that the desalination process must be stopped and the apparatus cleaned or replaced.
These salt precipitates are commonly known as scale and scale control research has been based upon the use of chemical agents, usually polymers, to control scale deposition although some have included surfactants. A common use-trait of most control agents is the very low ratio of agent to inorganic salt necessary to contain and retard precipitation. Thus, it is inferrable that the control effect is caused by complexation at the crystal growth site which repells incoming nuclei and not chelation of the inorganic cations of the salt.
The most common polymers selected for use as scale control agents are polymethacrylic acid, polyacrylic acid and polymaleic acid and their copolymers with monomers such as acrylamide and vinyl acetate. Some of the more recently developed scale control polymers are Hydrolyzed Polymaleic Anhydride (U.S. Pat. No. 3,810,832); Maleic Anhydride-Vinyl Acetate Copolymer (U.S. Pat. No. 3,715,307); Hydrolyzed Copolymer of Maleic Anhydride and Monoethylenically Unsaturated Monomer or Mixtures Thereof (GB 1414918); Maleic Anhydride Copolymers and Terpolymers (Dutch Pat. Application O.I.N. 7506874); Acrylic Acid Polymers (U.S. Pat. No. 3,514,376); Methacrylic Acid Polymer (U.S. Pat. No. 3,444,054); Maleic Acid Copolymers (U.S. Pat. No. 3,617,577); Styrene-Maleic Anhydride Copolymers (U.S. Pat. No. 3,289,734); and Polyacrylic Acid (U.S. Pat. No. 3,293,152).
A comparison of the nature of the polymer and the scale control activity as described by the prior art given supra, suggests that lowering the molar ratio of the maleic anhydride moiety to other monomer moieties in copolymers or terpolymers so composed will decrease both the charge density and the scale control activity. For example, the scale control data recorded for the polymers of GB 1414918 reveals that the activity against calcium carbonate precipitation decreases when progressing through the set of activities associated with homomaleic anhydride polymer through maleic anhydride-acrylamide copolymer to maleic anhydride-vinyl acetate-ethyl acrylate terpolymer. The same trend is apparent from a correlation of the scale control activities of the polymers of Dutch application 7506874. When the ratio of maleic anhydride portion to other monomer portion in the test polymer formed according to that application is decreased, the activity against calcium carbonate precipitation decreases. Thus, the literature teaches that the use of non-carboxylic acid monomers in conjunction with maleic anhydride monomer to form a polymer of mixed composition will cause a decrease in the scale control activity of that polymer relative to hydrolyzed polymaleic anhydride.
As is true for most polymers, the method used in preparing the scale control polymer will influence its nature and activity independent of the relative ratios of monomers used. General methods for the preparation of scale control polymers are described in U.S. Pat. No. 3,755,264, and U.S. Pat. No. 3,359,246, in addition to the methods contained in the patents describing use, supra. Although this prior art gives a wealth of information on how to prepare maleic anhydride and/or acrylic acid polymers, in practice, these preparation methods often lead to uncontrollable polymerization rates, the Tromsdorff effect, gelling and gumming of the polymers during polymerization, failure to control the magnitude and range of polymer molecular weight, impractical long polymerization times, and the unsafe nature of the reaction.
In view of the methods of preparation and suggestions of activity of the prior art it is surprising to discover that an interplay of monomer composition, temperature and reaction solvent according to the present invention allows the preparation of novel granular maleic anhydride terpolymer which has a controlled molecular weight range and is low in maleic anhydride content but nevertheless has scale control activity approximating that of hydrolyzed polymaleic anhydride. The novel process can also be employed to prepare useful maleic anhydride copolymers which are granular and readily processed.